In the architecture of cellular wireless network, the network coverage is divided into a plurality of cells with limited radio transmission range. Neighboring cells use different radio resources, and the cells that are far away from each other can share the same radio resources. Therefore, the cellular architecture does well in solving the spectrum congestion problem by reusing the radio resources, which can boost the frequency utilization. However, reusing the radio resources results in the increasing complexity of radio resource management, for instance, if inter-cell handover is required when the UE is moving across cells border from one cell to another, the ongoing call should be transferred from the channel in the serving cell to another channel in the targeted cell in order to maintain the seamless service.
With the gradually mature of third generation (3G) mobile communication systems, it is an inevitable trend that the second generation (2G) and 3G mobile communication will coexist in the market, e.g., GSM/GPRS, WCDMA, cdma2000 will coexist with TD-SCDMA. In the case of multi-system coexistence, users always expect to acquire different services from different network systems, and therefore, a multi-mode UE which is able to communicate with multiple systems emerges. The multi-mode UE can not only perform inter-cell handover in the same system, but also inevitably perform handover among systems which are employing different RAT(Radio Access Technologies), to obtain expected service via handover between different network systems.
Generally, handover procedure can be divided into three phases: communicating UE performs handover measurement; the network makes handover decision based on the measurement results reported by the UE; and the UE fulfills the handover operation according to the commands sent from the network. The complexity of handover measurement is closely related to the type of the executed handover, e.g., the inter-RAT handover measurement, which happens for handover between different network system, is much more complicated than general inter-cell handover measurement on same-frequency.
Since inter-RAT system handover involves two mutually independent radio access systems, carrier frequencies, synchronization information and system information will thereafter be changed. Therefore, when the UE which is conducting communication (communicating UE) measures the signal quality of a target RAT system, i.e. performing handover measurement, the communicating UE firstly switches to the carrier frequency used in the cell of the target RAT system; next performs synchronization operation to measure the signal quality of the cell; then reads the system information of the target RAT system, and at last switches to the carrier frequency of current serving RAT system so as to report the measurement results to the network. All these operations will occupy the radio resources of the communicating UE and are also considerably time-consuming.
For TDD system, the complexity to perform handover measurement is particularly outstanding. Because TDD system employs different timeslots in a same sub-frame to convey uplink and downlink traffic data respectively, the number of idle timeslots in the sub-frame will be quite limited comparing to FDD system, it is much difficult for TDD system to offer sufficient resource and time to perform inter-RAT handover measurement. In particular, when communicating UE is performing high-speed data transmission, since the available spare resources is extremely scarce, the handover measurement will impose heavy burden on the communicating UE. At this time, if the communicating UE needs to carry out measurement on multiple neighboring cells, it will be further burdened.
In order to ensure the successful completion of inter-RAT handover, dual receiver approach is proposed in the prior art. That is, UE is equipped with two receiver branches, one of which is used to guarantee the current communication, and the other one is used to perform inter-RAT handover management independently. However, the prospect of the dual receiver approach is quite dim due to the over-high cost.
Moreover, 3GPP protocols also propose that two receiver chains are not necessary to be maintained simultaneously, and the idle timeslots of communicating UE can be utilized to perform handover measurement. Based on this, single receiver approach for inter-RAT handover is developed.
One single receiver approach allows the communicating UE to use compressed mode to acquire continuous idle timeslots. For example, in TDD system, communicating UE can improve the transmission rate of the payload data in timeslot through reducing the spreading factor, so as to obtain continuous idle timeslots; or through compressing timeslot occupation in over-punching style similar to the FDD to acquire continuous idle timeslots. Another single receiver approach is to change the uplink and downlink slot assignment in FDD system, that is, reducing the interval between uplink and downlink timeslots to enable communicating UE to get continuous idle timeslots. However, using compressed mode or changing the assignment of uplink and downlink timeslot will result in data speed decreasing or BER degrading, and consequently will pose negative impact on the quality of service (QoS).
In conclusion, since inter-RAT handover measurement imposes heavy burden on communicating UE, a new handover measurement approach is therefore needed to extricate the communicating UE from overloaded handover measurement.